This invention relates to electrically conductive polyimide compositions and to a process for making such compositions. More specifically, the invention relates to a conductive polyimide composition comprising a uniform mixture of a polyimide matrix and a conductive filler comprising both carbon black and graphite particles, and a process for producing the same.
Conductive polyimides containing carbon particles are well-known in the art. For example, Canadian Patent 708,896 discloses an electrically conductive polyimide composition prepared by blending conductive carbon particles, e.g., carbon blacks, in the precursor polyamide acid; shaping the particle containing polyamide acid into a structure; and then thermally converting the shaped structure into a polyimide containing the carbon particles. Graphite is not used in combination with the carbon black particles, however.
U.S. Pat. No.4,568,412 to Atkins et al. discloses a surface conductive shaped article consisting of a polyimide matrix containing carbon black or graphite. Surface conductivity of the article is obtained by exposing the surface of the article to an aqueous or alcoholic etchant solution of an alkali metal hydroxide. The etchant removes a surface layer of polyimide thereby increasing the surface conductivity of the article. The patent does not suggest using a combination of carbon black and graphite to improve surface conductivity but, rather, requires a caustic etching step to obtain such an improvement.
In the manufacture of thin polyimide films having thicknesses of less than 10 mils containing conductive carbon black particles, the particles must first be sized to shapes that are small enough so as not to either foul the extrusion equipment or cause excessive surface roughness on the final polyimide film product. Since the manufacture of polyimide films typically proceeds through a polyamide acid procursor solution, it is most convenient to perform this particle classification in the polyamide acid precursor solvent, usually dimethyl acetamide, prior to blending the carbon black particles into the polyamide acid precursor solution. However, polar organic solvents, such as dimethyl acetamide, when used to mill carbon blacks have poor grinding efficiency and promote reagglomeration of the particles thereby causing thickening of the carbon particle solvent slurry. This effect is particularly pronounced with the more highly conductive carbon blacks which generally have large surface areas, ultimate particle sizes in the less than 20 millimicron range and tend to form long and often three dimensional clusters thereby increasing solution viscosity even at low concentrations. Thus, a moderately conductive carbon black having a nominal surface area of 25 m.sup.2/ g can be milled in dimethyl acetamide at a solids concentration of 18% to provide a polyimide film. Consequently for the final film to have a volume resistivity of 1 ohm-cm, high loadings of carbon black on the order of 25% are required. On the other hand, a highly conductive carbon black having a nominal surface area of 1250 m.sup.2/ g can be milled only at concentrations of less than 4% solids in dimethyl acetamide and reagglomeration is many times more rapid thereby resulting in slurries of poor quality. Consequently, polyamide acid solutions containing such highly conductive carbon blacks must be diluted to such high levels with the carbon black solvent slurry that the resulting casting dope is difficult to process due to low solution viscosity.
In contrast to carbon blacks which have a millimicron range of ultimate particle size, graphite particles have particle sizes in the micron range. Thus, graphite particles can easily be slurried in dimethyl acetamide, but concentrations needed to provide polyimide films having high conductivity are so large that the resultant films have poor structural integrity.
It has now been found that conductive polyimides containing both carbon black and graphite can be obtained by milling a large surface area, small particle size, highly conductive carbon black with large particle size, hard graphite particles in a polar solvent such as dimethyl acetamide; admixing the milled particle slurry with polyamide acid precursor; and thermally converting the polyamide acid containing the carbon black and graphite particles to polyimide. The resulting polyimide films have excellent electrical conductivity and due to the lower carbon loadings used, are tough and the surfaces are smooth. The use of graphite also increases slurry solids thereby increasing the solution viscosity of the casting dope.